a) Field of the Invention
The present invention relates to a method of scanning for exposure by a plurality of optical beams and an apparatus therefor, and in particular to the a method and apparatus in which a plurality of optical beams are generated using a multi-frequency acoustic optical element, which divides an incident optical beam into a plurality of portions according to an incident supersonic frequency, for simultaneous scanning for exposure.
b) Description of the Related Art
Conventionally, there has been proposed an apparatus for scanning optical beams, which allows stable and rapid reading or recording of an image, realized by using a plurality of laser beams formed by an optical modulating apparatus providing a multi-frequency acoustic optical element (AOM) (See, for example, Japanese Patent Publication 63-5741, Japanese Patent Laid-Opens 54-5455 and 57-41618, Japanese Patent Laid-Open 53-9856 and the like).
In the optical beam scanning apparatus such as a laser beam recording apparatus or the like, in which an image is recorded using such a multi-frequency acoustic optical element, a plurality of laser beams are arranged in an array so as to partially overlap with each other on a photosensitive surface. Then, the photosensitive surface is illuminated to carry out main scanning and sub-scanning of the laser beams by means of an optical scanning system which comprises a rotating polygon mirror, a galvanometer and the like to achieve scanning in a two-dimensional plane.
That is, the main scannings of the plurality of laser beams are simultaneously carried out by the reflection of the beams from the reflecting surface of the polygon mirror rotating at a rapid speed. In addition, these reflected laser beams are sub-scanned by being reflected by the galvanometer mirror rotated at a predetermined speed. By this sub-scanning procedure, the end portions of the plurality of laser beams are connected without any clearance so that a two-dimensional scanning is achieved to form an image on the two-dimensional plane.
Incidentally, recording materials for recording an image which are used in a laser beam recording apparatus or the like, may be roughly classified into: a silver salt film, the typical examples of which are a silver gelatin film and a thermally developed film (dry silver film), and a non-silver salt film, the typical example of which is a LDF (laser direct recording film) or the like. In an optical beam scanning apparatus such as a laser beam recording apparatus or the like, a silver salt film such as a dry silver film, which is treated using a dry method, is often used.
However, in an optical beam scanning apparatus such as a laser beam recording apparatus or the like using a group of laser beams, if silver salt film is used for the recording material, the density characteristic of the image can fluctuate due to the reciprocity law, and the reciprocity and multiple-exposure of the photosensitive material. That is, the fact that the end portions of the laser beams are each overlapped on the photosensitive material by sub-scanning means and that the end portions of the laser beams are further overlappingly exposed over time on the already exposed portion of the photosensitive material in order to carry out a subsequent recording (multiple exposure). For the silver salt film. It is conventionally known that the image density is increased by this multiple exposure. The reasons why this overlapping portion is increased in density are as follows: in the portion of the photosensitive material exposed by the low power portion (having low luminance) of the laser beam, which is a Gaussian beam, a sub-latent image and an initial image grow prior to the growth of the latent image associated with the prevailing density. If the laser beams are illuminated again after a predetermined time lapse to this sub-latent image and initial image portions on the photosensitive material, then the already exposed portion will correspond to the previously exposed portion and grow into a latent image by the repeated exposure, with the result that this overlapping portion (portion where the plurality of laser beams are connected) is increased in density.
In consequence, this increase in density of the overlapping portion on the photosensitive material causes an unevenness in the image density of the resulting film.